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Ring nebula - another first


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Quick process of Saturday nights attempt at the ring nebula. Turned out to be fairly easy to find though i could only get a hint that something was in the EP. A quick exposure at max iso revealed that i was dead on target ( for once ). It was smaller than i expected, almost tempting to barlow it, but not with my setup :)

This was about 18min integration, 56 x 20sec iso 1600, 30 dark, 30 bias, 20 flat, 20 dark flats.  I think i must have nudged / upset the polar finder alignment  because i really struggled to get even 20 sec without some trailing, and even with that the stars arn't as round as i would expect for such short subs. Was originally 80 subs but had to drop a few. 

Background looks a lot more red now that ive uploaded it too... anyways just glad to have gotten something with a hint of colour. I never really expect to get anything in London skies, but even with a non imaging setup and no clue of what im doing, its just great to get some hint of whats out there.

Suggestions, tips, thrown eggs welcome as always :)

Cheers

Mark

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Thanks Guys,
Yes! I believe the centre star is in fact the one that caused that lovely display. Just a little hot core giving off a lot of energy. Seem to remember something bout square laws and surface area vs luminosity or something. So i guess it must be really hot, or perhaps not that far away. I really should research my targets more :)

Nice work. Out of interest, do you use a light pollution filter?

I'm not sure about the trailling. Polar alignment could certainly be the cause, but I have heard the EQ3-2 is quite sensitive to balance. 

No light pollution filter. Have considered a cls clip but that would require longer exposures which i probably wouldnt achieve. I dont have any direct light pollution though just sky glow. Now that ive sorted out the flats ( before they were destroying my data) im just left with gradients. Especially when the moon is lurking behind a bush.
I agree about the PA, i think its a combination of bad balance and wind ( need to cut down on the cabbage before an imaging session :) ) There are a few subs that are perfect. THe finder isnt perfectly aligned though, it has slipped off centre, but not by much. No its down to the demands im putting on the mount at my FL i think.
Just for giggles, here is my my M51. IIRC its 1hr8m of 25sec subs ( that took a while to stack :) )
Pretty much just sticking my toe in the water with the equipment i have to see if its worth investing in a proper imaging rig. But i do wonder what will happen when i try longer exposures at a shorter FL??? :confused:  Will my skies hold up?
Cheers
Mark

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That's a darned good Ring nebula (a target I've never attempted myself.) It would be even better with a small amount of processing input. The background sky needs setting to a neutral colour. Personally I aim for 23/23/23 background per channel as measured in Photoshop. You can measure it using the Colour Sampler tool. Just by eye, I'd say you have a big bias towards red, here. Some people let the blue go higher than the other two. Personal taste.

For a general colour balance check you can open the image in Photoshop (and other programmes) and compare the histograms in each colour channel. The rule of thumb is to have the top left of the histogram peak aligned in each colour. You can only move the peaks to the left so you do this by moving the black point (left hand) slider to the right.

levels%20aligning-L.jpg**

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

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No giggles needed for your M51! That's a nice image. Just the gradient and noise to sort. Do you use Photoshop? If so, get the GradientXTerminator plug-in. It's marvelous, and balances the colour.

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Thanks for your kind comments guys!

Olly, thanks for the processing advice! Ive now reprocessed it, hopefully i got it right. I aligned the channels in the histogram just using the black point. Background wasnt near 23 so then i made adjustments to the mid point until the 11x11 sample was around 23 in all channels for the area i intended to crop. BIt of despeckle and unsharp mask. Cropped then reduced by 50%.

Seem to be getting a lot of colour noise so maybe i stretched it too far? Anyways it doesn't look red now. Very dark in laptop but ok on my monitor. Really need to download one of those colour / contrast chart things.

Alex, thanks, yes i use photoshop. Ive heard good things about GXtermitator, i think i'll stop sitting on the fence and just buy it! Now that the vignetting issues are fixed and my flats arnt killing my images ( managed to create a flat with TWO peak areas ??? ) Im finding that im left with nasty gradients...

Theres something im wondering about, anybody care to guess how long an exposure i'd get unguided at 1200mm fl on an neq6? and what about the addition of a 2x barlow? how long could i get with that?

Im pretty close to buying the neq6, probably next month. Gave up trying to decide whether to buy another scope, i dont really know what direction im going in so just going to use the PL until im sure. Curious about what to expect exposure time wise?

Cheers

Mark

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It depends on how well you've polar aligned. At that focal length, and especially doubled with a Barlow, I would think the exposures would be short. Unguided, I can get about 30s or about 45s near the zenith, and that's with a 1000mm scope. Mind you, I'm pretty poor at PA! The NEQ6 is more robust than my HEQ5 Pro so that might help.

Have you tried drift alignment? People talk about DARV By Robert Vice but I've never done it so can't advise, only that it's a little tricky at first, like most things, then it gets easier. It just takes time out of your imaging evening!

Edited by Astrosurf
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That makes sense, dunno why i thought i would get longer. Though in theory i wont have to throw away so many subs which is good. Looks like i'll have to go down the guiding route earlier than expected. But have the ASI cam so can try the finder scope method maybe. More stuff to research :)

I did take a look at the DARV method actually, I like the idea of it but yes it does seem like a fiddle and more time away from getting subs. Having said that, if im throwing away half my subs and not happy with the other half then its not time wasted.

Cheers

Mark

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No light pollution filter. Have considered a cls clip but that would require longer exposures which i probably wouldnt achieve. I dont have any direct light pollution though just sky glow. Now that ive sorted out the flats ( before they were destroying my data) im just left with gradients. Especially when the moon is lurking behind a bush.

Thanks, that's useful to know. I'm sticking to widefield imaging for the time being but further down the line I'd like a scope and imaging setup. That gives me a better idea of what might be possible from my local skies, where light pollution is fairly heavy.

Nice whirlpool as well, lots of detail there.

Yes! I believe the centre star is in fact the one that caused that lovely display. Just a little hot core giving off a lot of energy. Seem to remember something bout square laws and surface area vs luminosity or something. So i guess it must be really hot, or perhaps not that far away. I really should research my targets more  :)

Yes, the UV light from the hot white dwarf is ionising the surrounding gas, making it glow like a big streetlamp. The different elements emit at different wavelengths, the red is mostly hydrogen and nitrogen while the blue/green is mostly oxygen.

Edited by Knight of Clear Skies
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When you do things like sharpening, noise reducing and increasing colour saturation it is best to do them selectively, just to those parts of the image which need them. Ps has lots of selections tools, along with Layers, to make this easy. I use the colour select tool a great deal. So here, for instance, you could select just the background sky and reduce the saturation in it then run the Ps colour noise reduction filter.

Sharpening should be done only to regions of strong signal since it pumps up the noise like crazy. I also avoid sharpening stars 99 times out of 100.

Ths is a great first result.

Olly

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Thanks again for the processing advice Olly, I'll be brushing up on my ps skills then, starting with the selection tools for one and no more blanket sharpens either!

Best thing is, i can go back to all the other things i've imaged and apply what i've learned to them too. A definite win win situation :) Cheers for that!

Thanks, that's useful to know. I'm sticking to widefield imaging for the time being but further down the line I'd like a scope and imaging setup. That gives me a better idea of what might be possible from my local skies, where light pollution is fairly heavy.

Nice whirlpool as well, lots of detail there.

Yes, the UV light from the hot white dwarf is ionising the surrounding gas, making it glow like a big streetlamp. The different elements emit at different wavelengths, the red is mostly hydrogen and nitrogen while the blue/green is mostly oxygen.

Im constantly surprised by whats possible in heavy LP. Looking up at that orange/purple sky with its 20 or so visible stars you would be tempted just to go back inside and watch tv or whatever. Sticking an EP in doesnt help much for fuzzies, 45min looking at M1 and not seeing it was the start and end of my messier bagging experience. Dark adaption, pah! i dunno what it is, can read outside. I mean, other galaxies??? Have a tough time seeing the galaxy i live in.

But stick a camera on and open the shutter for 20 sec and the universe pops out. Im totally hooked. 

Hopefully at some point this year i'll be guiding and we will see whether a cls is required and how badly the lp limits exposure time. I'll keep you posted.

About the ring.... this from wikipedia: 
All the interior parts of this nebula have a blue-green tinge that is caused by the doubly ionized oxygen emission lines at 495.7 and 500.7 nm. These observed so-called "forbidden lines" occur only in conditions of very low density containing a few atoms per cubic centimeter. In the outer region of the ring, part of the reddish hue is caused by hydrogenemission at 656.3 nm, forming part of the Balmer series of lines. Forbidden lines of ionized nitrogen or [N II] contributes to the reddishness at 654.8 and 658.3 nm.

Is this a valid narrowband target? 

Cheers

Mark

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Mark, i have got lots of light pollution and i use the CLS filter clip and recently captured M57; i did 217 x 30 seconds (below). The CLS filter does mean you need to increase exposure time, number of exposures, ISO - whatever you want, to capture the same amount of data. But it is immensely powerful. I have also just got a baader Neodymium filter to test this out and that seems to block (slow) data acquisition less than the CLS filter, but i suspect wouldn't be as good at blocking sky glow for much longer exposures. I can't manage much longer exposures just yet though so not a problem!

2yba6ybu.jpg

Jd

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Wow thats a cracking image you got there Jd! Those colours are amazing!! Did you use a modded DSLR along with the CLS for that? Or is this the cls coupled with longer integration?

Whats amazing is that you did this at 2.7m and got great results using 30sec subs. This is all very encouraging.  How are you finding the eq6 at that FL, are you loosing many subs or is it just a few here and there?

Cheers

Mark

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The caveat should be that increasing ISO doesn't actually increase the amount of data, but you know what i mean.

Jd

Yep got you on that. I actually used iso1600 on the ring. Im usually 800, but transparency was good and the test at 1600 seemed ok, so i went for it.

Hopefully it will be ok tonight, going to gave a proper go at drift alignment. If i can push it up to 30sec then i'll be chuffed.

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It's an unmodded Canon 6D; remember unmodded cameras mostly just allow more hydrogen alpha light through, so won't help as much with galaxies or globs than with nebula (as far as i'm aware).

Just the 217 subs x 30 seconds. Darks and flats.

I binned three subs out of that run, one had an odd movement artefact (i suspect a gust of wind or a badger scratching itself on the mount), and deleted two due to satellite trails.

It's an AZEQ6. Yes, it seems ok at that focal length for my requirements. The peripheral (and some of the central)stars are not stunningly round, but that is multi factorial i think (non-flat field, poor balance when setting up, imperfect polar alignment, poor focus, etc). I don't want the hassle of guiding just yet, so happy to accept images of this quality. I did a handset polar alignment routine before i start the run, which i think is a really good way to tighten the polar alignment up.

I think if we are limited to 30 second subs, we need to accept we need several hundred of them to make sure the balance is massively in favour of signal rather than noise. There probably is an equation somewhere to show the relationship between sub length and total time for image acquisition; 12 x 5 minute subs (one hour data acquisition) under the same sky condition probably needs 4 hours of data acquisition using 30 second subs... Ok, an exaggeration but you know what i mean.

Jd

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Hi Jd,

THREE SUBS!!! out of 217  :shocked:  I think i get 3 that are good, 30% junk and the rest... well i'd rather junk em but then i'd have nothing :) The move to a more stable platform is going to be very welcome indeed :)

I think if we are limited to 30 second subs, we need to accept we need several hundred of them to make sure the balance is massively in favour of signal rather than noise. There probably is an equation somewhere to show the relationship between sub length and total time for image acquisition; 12 x 5 minute subs (one hour data acquisition) under the same sky condition probably needs 4 hours of data acquisition using 30 second subs... Ok, an exaggeration but you know what i mean.

Jd

I dont think thats an exaggeration, check out this site: http://www.pbase.com/samirkharusi/image/37608572

He has a lot of other great articles too. I need to re-read this, i read a ton of stuff like this when its cloudy, but promptly forget it all when the dust cap comes off.

I remember trying to figure out the sky glow using his method, but the clouds appeared. Based on slightly dodgy results i estimated around 17.5 mag/sq arc-sec. So very long integration times indeed. which is fair enough, but i cant be doing with dropping half my data cos its bad. For m51 i decided on 9 hours... i got 1hr 8min so far, 2 sessions i junked because of bad flats / poor transparency. The other 2 session were not so bad. But it certainly aint easy in uk skies, thats for sure :)

Cheers

Mark

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What focal length is your scope? What mount are you using? Can you eek out 60 second subs?

Why are so many of your subs poor? If you are wasting so many, could you drop your exposure time by 25%?

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Well its 1200mm on an eq 3-2. Most of the problem is wind i think. The PL is essentially a huge wind sail and it generates enough torque to flex the eq head.  Also any vibration such as walking near the scope ruins subs too. Balance is also an issue, particularly near the zenith as i have to be very careful not to let the ota bang into the mount legs, so that sometimes means not being as balanced as i would like. Not knocking it though, its great visual and planetary it will hold the target in a 300px box quite happily.

When i first got my polar scope i thought i was getting 60sec, but on close examination the stars were elongated. 10 or 15sec subs mean a gazillion files to stack so the compromise is 20 to 25 sec.

Im not annoyed about this, Its not an imaging set up and i never intended to get into AP. It just kinda happened. As far as im concerned everything ive done to date is just proof of concept.

The big question was, Is it worth trying to image in london? To which the answer is OH YES! It sure is.

So its time for the NEQ6 and hopefully i'll be throwing away the odd sub like you and keeping the rest.

Cheers

Mark

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